This invention relates to the use of spread spectrum or Code Division Multiple Access (CDMA) communications techniques in cellular radio telephone systems. More particularly, the invention relates to receivers used in direct sequence spread spectrum (DS-SS), or xe2x80x9ctraditionalxe2x80x9d direct-sequence CDMA systems.
CDMA or spread spectrum communications have been in existence since the days of World War II. Early applications were predominantly military oriented. However, today there has been increasing interest in using spread spectrum systems in commercial applications. Examples include digital cellular radio, land mobile radio, and indoor and outdoor personal communications networks. Commercial operations of the cellular telephone industry continue to grow, and users continue to demand flexible data transfer rates as a key feature in newer communications systems.
CDMA allows signals to overlap in both time and frequency, as illustrated in FIG. 1. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other. In a CDMA system, an information data stream (e.g., speech) to be transmitted is impressed upon a much higher bit rate data stream known as a spreading code signal, a signature sequence or a code sequence. The signature sequence, which has a random appearance, can be generated by a pseudorandom code generator, and replicated in an authorized receiver. The information data stream can be combined with the signature sequence by effectively multiplying the two bit streams together. Combining the higher bit rate signature sequence with the lower bit rate information data stream is called coding or spreading the information data stream signal. Each information data stream or channel is allocated a unique spreading code or signature sequence. A plurality of coded information signals are modulated and transmitted on a radio or carrier wave as a modulated composite signal. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. The modulated composite signal of multiple coded signals is received at a receiver and is demodulated into a baseband frequency. The demodulated composite signal, or baseband signal, can also be referred to as a complex signal because it typically contains both real and imaginary components. A coded signal is extracted and isolated from the demodulated composite signal by correlating the coded signal using the same signature sequence that was used to create the coded signal.
Typically, the information data stream and the signature sequence are binary with the bits of the signature sequence being known as xe2x80x9cchipsxe2x80x9d. In traditional direct-sequence CDMA or spread spectrum systems, a signature sequence having N chips is used to represent one bit or data symbol of the information data stream. An entire transmitted N-chip sequence is referred to as a transmitted symbol.
In particular, FIGS. 2 and 3 illustrate how information signals in a CDMA system are encoded and decoded. Two different data streams (a) and (d) are shown graphically in FIG. 2, and represent digitized information to be communicated over two separate communication channels as Signal 1 and Signal 2, respectively. Signal 1 is modulated using a unique signature sequence having a high bit rate, and is thereby encoded as shown in signal graph (b) of FIG. 2. The term xe2x80x9cbitxe2x80x9d refers to one digit of the information signal. The term xe2x80x9cbit periodxe2x80x9d refers to the time period between the beginning and the end of the bit signal. Accordingly, the chip period refers to the time period between the beginning and the end of one digit of the high rate signature sequence signal. The bit period is much greater than the chip period. The result of this modulation, which is essentially the product of the signature sequence and the data stream, is shown in signal graph (c) of FIG. 2. In Boolean notation, the modulation of two binary waveforms is essentially an exclusive-OR operation. A similar series of operations is carried out for Signal 2 as shown in signal graphs (d)-(f) of FIG. 2. In practice, of course, many more than two coded information signals are spread across the frequency spectrum available for cellular telephone communications.
Each coded signal is used to modulate an RF carrier using any one of a number of modulation techniques, such as Quadrature Phase Shift Keying (QPSK). Each modulated carrier is transmitted over an air interface. At a radio receiver, such as a cellular base station, all of the modulated carrier signals that overlap in the allocated frequency bandwidth are received together, and are effectively added to form a composite of the modulated carrier signals, or a composite transmission signal. The composite of modulated carrier signals is demodulated to the appropriate baseband frequency, and the result is a composite or sum of the individually coded signals. For example, signal graph (c) of FIG. 3 is a composite or sum of the individually coded signals of signal graphs (a) and (b) of FIG. 3, i.e., is a composite baseband signal. The composite baseband signal can have in-phase and out-of-phase components, i.e., real and imaginary components, and can also be referred to as a complex baseband signal.
The original data streams can be extracted or decoded from the composite baseband signal. For example, signal 1 can be decoded by multiplying the composite baseband signal in the signal graph (c) of FIG. 3 with the unique signature sequence used originally to encode signal 1, as shown in the signal graph (d) of FIG. 3. The resulting signal is analyzed to decide the polarity (high or low, +1 or xe2x88x921, xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) of each information bit period of the signal.
These decisions can be made by taking an average or majority vote of the chip polarities during one bit period. Such xe2x80x9chard decisionxe2x80x9d making processes are acceptable as long as there is no signal ambiguity. For example, during the first bit period in the signal graph (f), the average chip value is +0.67 which readily indicates a bit polarity +1. Similarly, during the subsequent bit period, the average chip value is xe2x88x921.33. As a result, the bit polarity was most likely a xe2x88x921. Finally, in the third bit period, the average is +0.80 which indicates a bit polarity of +1. However, whenever the average is zero, the majority vote or averaging test fails to provide an acceptable polarity value.
In most situations, a xe2x80x9csoft decisionxe2x80x9d making process is used to determine the bit polarity. For example, an analog voltage proportional to the received signal after despreading can be integrated over the number of chip periods corresponding to a single information bit. The sign or polarity of the net integration result indicates that the bit value is a +1 or xe2x88x921.
CDMA receivers often contain a RAKE receiver. In mobile communication systems, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion, caused for example by signal reflections from large buildings or nearby mountain ranges. Multipath dispersion occurs when a signal proceeds to the receiver along not one but many paths so that the receiver hears many echoes having different and randomly varying delays and amplitudes. Typically a RAKE receiver xe2x80x9crakesxe2x80x9d all the multipath contributions together. A CDMA RAKE receiver individually detects each echo signal using a correlation method, corrects for different time delays, and adds the detected echo signals algebraically (with the same sign).
Sliding correlators can be used in spread spectrum or CDMA receivers to perform the correlation/extraction process, and are typically capable of doing so relatively quickly. In particular, a conventional sliding correlator can correlate the baseband signal with a portion of a signature sequence used to spread the signal. The signature sequence portion is also known as a local code section, and is correlated at a rate equal to the chip rate. The chip rate is the inverse of a time period of a chip in the signature sequence as broadcasted. Sliding correlators are sometimes known as matched filters, since they can be used to search for a match between a received and sampled baseband signal and section of a signature sequence.
Sliding correlators can be used, for example, in the conventional spread spectrum receiver shown in FIG. 4, which includes an antenna 40, Radio Frequency (RF) section 42, and a baseband processor 44. FIG. 5 shows an internal configuration of the baseband processor 44 of FIG. 4, wherein a complex baseband signal 50 is input to a sampler 52. The sampler 52 samples the baseband signal at a specified rate. The baseband signal can be sampled, for example, twice per chip. The samples are provided to a RAKE receiver 54, a set of early-late gates 58 and an initial acquisition and search unit 56. The RAKE receiver 54 detects the signal, and the early-late gates 58 provide a multipath delay estimate to the RAKE receiver. The unit 56 provides initial acquisition and search functions that are typically necessary in a spread spectrum receiver. A signature sequence is typically broadcast from beginning to end, and then repeated. The signature sequence can be unmodulated, partially modulated, or fully modulated by information symbols, depending on whether the system has data only, pilot symbols, or a pilot channel. Initial acquisition refers to a process of generally determining which portion of the signature sequence is currently being broadcast. In contrast, searching refers to a process of precisely determining which portion of the signature sequence is currently being broadcast, so that data can be extracted from the broadcast signal. A sliding correlator can reside, for example, in the unit 56.
FIG. 6 shows a block diagram of a conventional sliding correlator 600 having a delay line 604, delay taps 606, multiplying taps 608, a summer 610, and a magnitude square 612. The baseband signal is sampled uniformly with samples 710, wherein each chip 712 is sampled twice, as shown in FIG. 7. The baseband samples 710 are provided as an input signal 602 to the delay taps 606. A signal is uniformly sampled when the time between samples is constant. The series of delay taps 606 forms the delay line 604, so that the delay line 604 effectively functions as a first-in-first-out (FIFO) register, or queue. Since there are two samples per chip, the sample values move from one delay tap to the next delay tap at twice the chip rate. The sample values present in the delay taps 606 are provided to the multiplying taps 608, where they are multiplied by tap coefficients that correspond to a designated spreading code or signature sequence of a signal from which data is to be extracted. The resulting values are then added in the summer 610. An output of the summer 610 is provided to magnitude square 612, which outputs a number that is the square of a magnitude of the output from the summer 610. The output of the magnitude square 612 can be used to identify and acquire a signal that was encoded with the designated signature sequence.
Despite the numerous advantages afforded by CDMA, the capacity of conventional CDMA systems is limited by the decoding process. Because many different user communications overlap in time and frequency, the task of correlating the correct information signal with the appropriate user is complex. Although sliding correlators are relatively fast, they are also costly to implement because they require a relatively large number of gates on a Very Large Scale Integrated (VLSI) chip and consume relatively large amounts of power. Accordingly, sliding correlators having an improved cost-to-utility ratio are desirable.
According to an exemplary embodiment of the invention, a flexible sliding correlator for use in a spread spectrum receiver divides baseband signal samples into different groups, associates each group with a different section or portion of a spreading code or signature sequence, and combines the signal samples with corresponding values in the signature sequence section. The groupings and signature sequence sections can be changed during operation of the receiver to maximize performance of the receiver under different or changing conditions. In addition, the sample and signature sequence value combinations can be further combined in different ways, and the further combinations can be changed during operation of the receiver. The flexible sliding correlator can be adaptively configured during operation of the receiver by changing the grouping of the baseband samples, the signature sequence sections used to process each group, and the combination of group processing results. According to another aspect of the invention, the baseband signal can be sampled either uniformly or non-uniformly. The phase and frequency of the baseband sampling can be adjusted during operation of the receiver so that samples are taken very close to the optimum sampling position, at the peak of a chip waveform in the baseband signal.